This article documents notes on the process for upgrading Zulip to
new major versions of Django. Here are the steps:

Carefully read the Django upstream changelog, and gitgrep to
check if we’re using anything deprecated or significantly modified
and put them in an issue (and then starting working through them).
Also, note any new features we might want to use after the upgrade,
and open an issue listing them;
example.

Start submitting PRs to do any deprecation-type migrations that work
on both the old and new version of Django. The goal here is to have
the actual cutover commit be as small as possible, and to test as
much of the changes for the migration as we can independently from
the big cutover.

Check the version support of the third-party Django packages we use
(gitgrepdjangorequirements/ to see a list), upgrade any as
needed and file bugs upstream for any that lack support. Look into
fixing said bugs.

Look at the pieces of Django code that we’ve copied and then
adapted, and confirm whether Django has any updates to the modified
code we should apply. Partial list:

PasswordResetForm and any other forms we import from
django.contrib.auth.forms in zerver/forms.py (which has all of
our Django forms).

AsyncDjangoHandlerBase in zerver/tornado/handlers.py is a very
small patch on base.BaseHandler; see the comments with BYZULIP for our changes (unfortunately now more different due to
style differences and mypy annotations). This configurability
should also be contributed to Django upstream.